1. Field of the Invention
This invention relates to a radiation detector. It is particularly, but not exclusively, related to a microbridge for a thermal radiation detector.
2. Discussion of Prior Art
Thermal radiation detectors are commonly used to detect infra-red (IR) radiation. Although this can be defined as being in the wavelength range 0.7 xcexcm to 1 mm, the range 8 to 14 xcexcm is of particular interest. This corresponds to a peak in black body radiation emitted by a body at 300 K. It also corresponds to a window in transmissivity of the atmosphere, that is a relatively transmissive wavelength range between regions of the electromagnetic spectrum in which there is significant attenuation of radiation by the atmosphere.
A known thermal detector comprises an array of microbridges mounted on a substrate. Each microbridge has an active element in planar form sandwiched between upper and lower electrodes, both of which are also in planar form. A cavity is defined between the substrate and the lower electrode so as to provide thermal isolation between the active element and the substrate. The active elements comprise a ferroelectric material such as PZT. The use of the pyroelectric effect in thermal detectors is well established. In one embodiment a known thermal detector has a two-dimensional array of microbridges coupled to a readout circuit to give a staring imaging device. Such a thermal detector is described in GB 9625722.5.
It is necessary for a microbridge to have a high degree of thermal isolation from its substrate. This can be achieved by the microbridge being connected to the substrate only by very thin links. Separation of the bulk of a microbridge from the substrate is achieved by fabricating the microbridge over an etchable sacrificial layer and etching it away after the microbridge has been formed.
Generally, the substrate comprises a silicon readout circuit. The electrodes of each microbridge are connected to separate contact points on the readout circuit.
Thermal radiation, such as intra-red radiation, incident on and absorbed by each microbridge produces an electrical response in each active element in which the amount of charge induced across an active element depends on temperature changes. The capacitance across the active element remains substantially constant and so the voltage across the active element also depends on its temperature. Therefore measurement of the voltage across a microbridge provides an indication of its temperature changes which provides an indication of changes in the intensity of thermal radiation falling on it.
For optimum performance, the thermal properties of a microbridge are designed to match its operating frequency, or the centre of the range of operating frequencies. This is achieved by tailoring the thermal time constant of the microbridge, that is the ratio of the heat capacity to the thermal conductance, to be similar to the period of the operating frequency. This provides low thermal losses. The performance can be further improved by reducing both the thermal capacity and the conductance, because this results in an increased temperature change for a given change in the level of absorbed radiation. One method of reducing the heat capacity is to reduce the volume of the active element. A reduced heat capacity is only beneficial if: the amount of radiation absorbed is not significantly reduced; the thermal conductance is reduced so as to keep the thermal time constant approximately constant; and the heat capacity of the active element is similar to or larger than the heat capacity of the rest of the microbridge structure. It is also desirable that the dimensions of the active element are selected such that its electrical impedance is matched to the input impedance of the silicon readout circuit.
If the active element is made thinner, absorption of radiation by it becomes inefficient when its thickness, divided by its refractive index, is significantly thinner, than a quarter of the wavelength of incident radiation. This defines a thickness beyond which it is disadvantageous to make the active element thinner. Furthermore there is a greater likelihood of electrical breakdown of the active element when a high electrical field is applied either for poling or for operation under bias.
According to the invention there is provided a radiation detector comprising a substrate a microbridge supported above the substrate by electrodes so that a cavity is present between the microbridge and the substrate the microbridge having a collecting area a region of which is occupied by an active area, said active area comprising detector means for converting received radiation into an electrical response characterised in that radiation absorbed by at least one region of the collecting area which is not occupied by the active area is converted into heat energy and transmitted to the active area by conduction.
Preferably the detector comprises a plurality of microbridges. Conveniently they are disposed in an array on the substrate.
Preferably the substrate is a semiconductor substrate. Most preferably it is a silicon readout chip. The chip may comprise, or be connected to, processing means which processes the electrical response of a microbridge to determine radiation intensity incident on the microbridge. Conveniently the electrical response is a charge, a voltage or both. The electrical response may change in response to radiation intensity incident on the microbridge.
Preferably the processing means uses information derived from radiation incident on the array of microbridges to produce an image.
Preferably the active area has a thickness which is a quarter of the wavelength of radiation to be detected.
Preferably the substrate is provided with a reflective surface which reflects incident radiation which passes through the microbridge. Most preferably the reflective surface is planar. Preferably there is a constant separation between the collecting area and the substrate. Advantageously the separation is constant across the whole of the collecting area.
Preferably the detector detects thermal radiation. It may be a thermal imaging device. Preferably it detects radiation in the IR part of the electromagnetic spectrum. Most preferably it detects radiation in the ranges 3 to 5 xcexcm and 8 to 14 xcexcm. Alternatively the microbridge is a detector element which detects electromagnetic radiation in parts of the spectrum other than IR, such as visible and ultra-violet (UV) radiation.
Preferably the radiation detector detects radiation by using the pyroelectric effect.
Preferably the active area comprises a ferroelectric material. The ferroelectric material may comprise lead zirconium tantalate (PZT), lead lanthanum titanate (PLT) or lead scandium tantalate (PST). Conveniently the layer of active material may be formed by deposition from solution and/or a sol-gel technique.
Preferably substantially the majority of heat energy received by the active area is provided by conduction.